U.S. patent application number 17/477693 was filed with the patent office on 2022-01-06 for mode division multiplexer, mode division multiplexing system, mode division demultiplexing system, and communications system.
This patent application is currently assigned to HUAWEI TECHNOLOGIES CO., LTD.. The applicant listed for this patent is HUAWEI TECHNOLOGIES CO., LTD.. Invention is credited to Ji Luo, Yong Wang, Xiang Yin.
Application Number | 20220006555 17/477693 |
Document ID | / |
Family ID | 1000005899276 |
Filed Date | 2022-01-06 |
United States Patent
Application |
20220006555 |
Kind Code |
A1 |
Yin; Xiang ; et al. |
January 6, 2022 |
MODE DIVISION MULTIPLEXER, MODE DIVISION MULTIPLEXING SYSTEM, MODE
DIVISION DEMULTIPLEXING SYSTEM, AND COMMUNICATIONS SYSTEM
Abstract
This application provides a mode division multiplexer, which
includes a metasurface of an electromagnetic resonance unit that
has a plurality of sub-wavelengths disposed in an array. The
electromagnetic resonance unit is configured to perform phase
modulation on a beam transmitted to the electromagnetic resonance
unit, to convert a spatial mode order of the beam. Because a size
of the electromagnetic resonance unit is a sub-wavelength, and a
pixel size of the electromagnetic resonance unit is smaller than a
pixel size of a spatial light modulator in the prior art, crosstalk
between different spatial modes after phase modulation performed by
the mode division multiplexer is comparatively low. In this way,
the crosstalk is comparatively small when beams in different
spatial modes are multiplexed into a few-mode/multi-mode fiber. The
mode division multiplexer in this application can implement
polarization-independent phase modulation.
Inventors: |
Yin; Xiang; (Shanghai,
CN) ; Wang; Yong; (Shanghai, CN) ; Luo;
Ji; (Moscow, RU) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
HUAWEI TECHNOLOGIES CO., LTD. |
Shenzhen |
|
CN |
|
|
Assignee: |
HUAWEI TECHNOLOGIES CO.,
LTD.
Shenzhen
CN
|
Family ID: |
1000005899276 |
Appl. No.: |
17/477693 |
Filed: |
September 17, 2021 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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PCT/CN2020/080000 |
Mar 18, 2020 |
|
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17477693 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04J 14/04 20130101;
H04B 10/2581 20130101; G02B 6/2848 20130101; G02B 6/29304
20130101 |
International
Class: |
H04J 14/04 20060101
H04J014/04; G02B 6/28 20060101 G02B006/28; H04B 10/2581 20060101
H04B010/2581; G02B 6/293 20060101 G02B006/293 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 22, 2019 |
CN |
201910235056.8 |
Claims
1. A mode division multiplexer, comprising a first collimator, a
second collimator, and a mode division multiplexing module located
between the first collimator and the second collimator, wherein the
mode division multiplexing module is configured to perform phase
modulation a plurality of times on each of a plurality of
independent beams incident through the first collimator, to
implement mode conversion and beam combination, such that a beam
output through the mode division multiplexing module is incident on
the second collimator; and the mode division multiplexing module
comprises a metasurface, wherein the metasurface comprises a
plurality of sub-wavelength electromagnetic resonance units
disposed in an array, and each of the plurality of sub-wavelength
electromagnetic resonance units is configured to perform the phase
modulation on a beam transmitted to the electromagnetic resonance
unit, to convert a spatial mode of the beam transmitted to the
electromagnetic resonance unit.
2. The mode division multiplexer according to claim 1, wherein the
mode division multiplexing module comprises two reflectors disposed
oppositely, the beam is emitted after being reflected by the
reflectors a plurality of times between the two reflectors, and at
least one of the two reflectors is the metasurface, and the phase
modulation is performed once each time the beam is reflected by the
metasurface.
3. The mode division multiplexer according to claim 2, wherein one
of the two reflectors is the metasurface, the other reflector is a
mirror reflector, a reflection surface of the mirror reflector
faces the metasurface.
4. The mode division multiplexer according to claim 2, wherein both
of the two reflectors are metasurfaces, and the phase modulation is
performed once each time the beam is reflected on either of the two
metasurfaces.
5. The mode division multiplexer according to claim 3, wherein the
metasurface comprises a metal substrate, a dielectric layer, and an
array layer that are disposed sequentially through stacking; the
array layer comprises a plurality of metal blocks disposed in an
array; the metal substrate comprises a plurality of first
sub-blocks disposed in an array; the dielectric layer comprises a
plurality of second sub-blocks disposed in an array; the plurality
of first sub-blocks are in a one-to-one correspondence with the
plurality of second sub-blocks, and one metal block is stacked on
each second sub-block; and each first sub-block, each second
sub-block, and the metal block stacked on the second sub-block form
each electromagnetic resonance unit.
6. The mode division multiplexer according to claim 5, wherein the
metal substrate is an aluminum substrate, the dielectric layer is a
silicon dioxide layer, and the metal block is a gold block.
7. The mode division multiplexer according to claim 1, wherein the
mode division multiplexing module comprises a plurality of
metasurfaces, the plurality of metasurfaces are disposed in
parallel and spaced from each other, a beam sequentially passes
through the plurality of metasurfaces, and the phase modulation is
performed once each time the beam passes through one of the
metasurfaces.
8. The mode division multiplexer according to claim 7, wherein the
metasurface comprises a substrate, an array of the electromagnetic
resonance units is disposed on a surface of the substrate, and a
refractive index of a dielectric material forming the
electromagnetic resonance unit is greater than 2.
9. The mode division multiplexer according to claim 8, wherein the
substrate is a silicon dioxide substrate, and the electromagnetic
resonance unit is a silicon nanocube.
10. The mode division multiplexer according to claim 1, wherein a
size of each electromagnetic resonance unit on the metasurface
matches a phase change value of the beam before and after the phase
modulation performed by the electromagnetic resonance unit; and
distribution of the electromagnetic resonance units of different
sizes matches light field distribution of the beam on the
metasurface.
11. The mode division multiplexer according to claim 10, wherein
mode conversion satisfies the following formula:
O=F.sub.L2T.sup.n.sub.a.times.bF.sub..DELTA.xnT.sup.n-1.sub.a.times.b
. . .
F.sub..DELTA.x2T.sup.2.sub.a.times.bF.sub..DELTA.x1T.sup.1.sub.a.times.-
bF.sub.L1I I is an input optical field distribution matrix, and O
is an output optical field distribution matrix; L1 is a distance of
transmitting a beam from the first collimator to the mode division
multiplexing module, L2 is a distance of transmitting the beam from
the mode division multiplexing module to the second collimator, and
F.sub.L1 and F.sub.L2 respectively indicate Fresnel diffraction
matrices corresponding to transmission distances L1 and L2;
F.sub..DELTA.xi indicates a Fresnel diffraction matrix
corresponding to a transmission distance .DELTA.xi, wherein the
distance .DELTA.xi is a distance of transmitting the beam after
i.sup.th phase modulation and before (i+1).sup.th phase modulation
of the beam, i=1, 2, . . . , n, and n is a natural number greater
than 1; T.sup.i.sub.a.times.b is a unitary matrix corresponding to
the i.sup.th phase modulation performed on the metasurface,
a.times.b indicates that each phase modulation is completed by
using a.times.b pixels, and each pixel has one or more
electromagnetic resonance units disposed in an array; and an area
of the metasurface comprising the a.times.b pixels is greater than
an effective light spot area of the metasurface to which the beam
is transmitted; and a unitary matrix T.sup.i.sub.a.times.b
corresponding to each metasurface phase modulation is obtained by
using the determined I, O, F.sub.L1, F.sub.L2, and F.sub..DELTA.xi,
to obtain the sizes and the distribution of the electromagnetic
resonance units on the metasurface.
12. The mode division multiplexer according to claim 1, wherein the
mode division multiplexer is a linearly polarized mode multiplexer,
and a quantity of (m+1) times of phase modulation performed by the
mode division multiplexing module on a beam and a quantity N of
multiplexing modes of the linearly polarized mode multiplexer
satisfies a formula: m=2N.
13. The mode division multiplexer according to claim 1, wherein the
mode division multiplexer is a non-linearly polarized mode
multiplexer, and a quantity of times of phase modulation performed
by the mode division multiplexing module on a beam is positively
related to a quantity of multiplexing modes of the non-linearly
polarized mode multiplexer.
14. The mode division multiplexer according to claim 1, wherein
both the first collimator and the second collimator are
metasurfaces.
15. The mode division multiplexer according to claim 1, wherein the
electromagnetic resonance unit enables an adjustment range of a
phase change value generated for a beam transmitted to the
electromagnetic resonance unit to be 0 to 2.pi..
16. The mode division multiplexer according to claim 1, further
comprising an assembly component, wherein the first collimator, the
second collimator, and the mode division multiplexing module are
all assembled into the assembly component.
17. A mode division multiplexing system, comprising an input fiber,
an output fiber, and the mode division multiplexer according to
claim 1, wherein the input fiber and the output fiber are
respectively connected to two opposite sides of the mode division
multiplexer; the input fiber is close to a side of a first
collimator in the mode division multiplexer, the output fiber is
close to a side of a second collimator in the mode division
multiplexer, and the system is configured to sequentially transmit
a beam from the input fiber to the mode division multiplexer, and
then to the output fiber; the input fiber is configured to provide
a plurality of channels, and each channel transmits one independent
beam to the mode division multiplexer; the mode division
multiplexer is configured to perform phase modulation a plurality
of times on each of a plurality of independent beams input through
the input fiber, so that spatial modes of the plurality of beams
are respectively converted into spatial modes that match different
fiber modes in the output fiber, and combine the plurality of
beams, wherein beams transmitted through different channels are
converted into beams with different spatial modes through the mode
division multiplexer.
18. A mode division demultiplexing system, comprising an input
fiber, an output fiber, and the mode division multiplexer according
to claim 1, wherein the input fiber and the output fiber are
respectively connected to two opposite sides of the mode division
multiplexer; the input fiber is close to a side of a second
collimator in the mode division multiplexer, the output fiber is
close to a side of a first collimator in the mode division
multiplexer, and the system is configured to sequentially transmit
a beam from the input fiber to the mode division multiplexer, and
then to the output fiber; the input fiber supports a plurality of
different fiber modes, and the different fiber modes are used to
carry different signals and transmit the signals to the mode
division multiplexer; the mode division multiplexer is configured
to perform mode conversion on beams in different spatial modes and
perform beam splitting, so that the different spatial modes of the
beam are converted into spatial modes that match an output fiber
mode, and perform beam splitting on a beam emitted from the input
fiber; and the output fiber is configured to receive and transmit
the split beam emitted through the mode division multiplexer,
wherein the output fiber comprises a plurality of channels, and
each channel is configured to transmit one independent split
beam.
19. A communications system, comprising a mode division
multiplexing system and a mode division demultiplexing system,
wherein the mode division multiplexing system comprising an input
fiber, an output fiber, and the mode division multiplexer according
to claim 1, wherein the input fiber and the output fiber are
respectively connected to two opposite sides of the mode division
multiplexer; the input fiber is close to a side of a first
collimator in the mode division multiplexer, the output fiber is
close to a side of a second collimator in the mode division
multiplexer, and the mode division multiplexing system is
configured to sequentially transmit a beam from the input fiber to
the mode division multiplexer, and then to the output fiber; the
input fiber is configured to provide a plurality of channels, and
each channel transmits one independent beam to the mode division
multiplexer; the mode division multiplexer is configured to perform
phase modulation a plurality of times on each of a plurality of
independent beams input through the input fiber, so that spatial
modes of the plurality of beams are respectively converted into
spatial modes that match different fiber modes in the output fiber,
and combine the plurality of beams, wherein beams transmitted
through different channels are converted into beams with different
spatial modes through the mode division multiplexer; wherein the
mode division demultiplexing system comprising an input fiber, an
output fiber, and the mode division multiplexer according to claim
1, wherein the input fiber and the output fiber are respectively
connected to two opposite sides of the mode division multiplexer;
the input fiber is close to a side of a second collimator in the
mode division multiplexer, the output fiber is close to a side of a
first collimator in the mode division multiplexer, and the mode
division demultiplexing system is configured to sequentially
transmit a beam from the input fiber to the mode division
multiplexer, and then to the output fiber; the input fiber supports
a plurality of different fiber modes, and the different fiber modes
are used to carry different signals and transmit the signals to the
mode division multiplexer; the mode division multiplexer is
configured to perform mode conversion on beams in different spatial
modes and perform beam splitting, so that the different spatial
modes of the beam are converted into spatial modes that match an
output fiber mode, and perform beam splitting on a beam emitted
from the input fiber; and the output fiber is configured to receive
and transmit the split beam emitted through the mode division
multiplexer, wherein the output fiber comprises a plurality of
channels, and each channel is configured to transmit one
independent split beam; wherein the output fiber in the mode
division multiplexing system is the input fiber in the mode
division demultiplexing system, and the communications system is
configured to transmit a beam between the mode division
multiplexing system and the mode division demultiplexing system.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of International
Application No. PCT/CN2020/080000, filed on Mar. 18, 2020, which
claims priority to Chinese Patent Application No. 201910235056.8,
filed on Mar. 22, 2019. The disclosures of the aforementioned
applications are hereby incorporated by reference in their
entireties.
TECHNICAL FIELD
[0002] This application relates to the field of communications
technologies, and in particular, to a mode division multiplexer, a
mode division multiplexing system, a mode division demultiplexing
system, and a communications system.
BACKGROUND
[0003] In recent years, with rapid development of emerging services
such as high-definition video, the internet of things, and cloud
computing, people have increasing demand for bandwidth. Currently,
a single-mode fiber has approached a theoretical transmission limit
thereof, and a space division multiplexing technology is expected
to greatly improve a capacity of fiber optic communications by
using a new space division multiplexing dimension, to break the
theoretical transmission limit of the single-mode fiber. As a type
of space division multiplexing technology, mode division
multiplexing has attracted wide attention in recent years. Mode
division multiplexing uses an independent orthogonal mode in a
few-mode/multi-mode fiber as a signal transmission channel to
improve the capacity of fiber optic communications.
[0004] A mode division multiplexer is a type of essential component
in a mode division multiplexing (MDM) system. The mode division
multiplexer is configured to multiplex independent signals in a
single-mode fiber array or a multi-core fiber into corresponding
modes in a few-mode/multi-mode fiber to implement a mode
multiplexing function, or demultiplex signals in different modes
that are transmitted in a few-mode/multi-mode fiber into a
single-mode fiber or a multi-core fiber for independent
transmission. Up to now, reported mode division multiplexers
include the use of a binary phase plate, a long-period fiber
grating, a fiber coupler, a photonic lantern, a multi-plane light
converter, and the like. Multi-plane light conversion is considered
as a potentially valuable application technology. However, in an
existing multi-plane light converter, phase plates or spatial light
modulators are usually disposed on a plurality of discrete planes
to implement continuous conversion of a spatial optical
transmission wavefront. However, because of comparatively large
pixel sizes of the spatial light modulator and the phase plate,
there are problems such as low resolution and comparatively high
crosstalk in a wavefront conversion process (namely, a spatial mode
conversion process of a beam).
SUMMARY
[0005] This application provides a mode division multiplexer, to
provide a mode division multiplexer that can implement high
resolution, low crosstalk, and low insertion loss. This application
further provides a mode division multiplexing system, a mode
division demultiplexing system, and a fiber optic communications
system.
[0006] According to a first aspect, this application provides a
mode division multiplexer. The mode division multiplexer includes a
first collimator, a second collimator, and a mode division
multiplexing module located between the first collimator and the
second collimator. Phase modulation is performed, by the mode
division multiplexing module, for a plurality of times on a
plurality of independent beams incident through the first
collimator, to implement mode conversion and beam combination. A
beam output through the mode division multiplexing module is
incident on the second collimator.
[0007] The mode division multiplexing module includes a
metasurface, the metasurface includes a plurality of sub-wavelength
electromagnetic resonance units disposed in an array, and the
electromagnetic resonance unit is configured to perform phase
modulation on a beam transmitted to the electromagnetic resonance
unit, to convert a spatial mode of the beam transmitted to the
electromagnetic resonance unit. In this application, a period of
the electromagnetic resonance unit is approximately half a
wavelength of the beam, so that higher-order diffraction of the
beam on the metasurface can be suppressed, thereby improving
efficiency of wavefront conversion performed on the beam on the
metasurface.
[0008] In this application, the mode division multiplexing module
includes the metasurface, the electromagnetic resonance units
disposed in an array are disposed on the metasurface, and the
electromagnetic resonance unit can change a phase, an amplitude, a
polarization state, and the like of the beam passing through the
electromagnetic resonance unit. A spatial mode of a beam
transmitted to the metasurface can be converted through a plurality
of times of phase modulation performed on the metasurface. In this
application, phase modulation of a beam is based on an equivalent
refractive index change caused by electrical resonance and magnetic
resonance. Specifically, when the beam passes through the
electromagnetic resonance unit, electrical resonance and magnetic
resonance are generated with the electromagnetic resonance unit.
Therefore, when the beam is transmitted through the electromagnetic
resonance unit, an equivalent refractive index of the
electromagnetic resonance unit changes, and consequently, a phase
of the beam after the beam is emitted through the electromagnetic
resonance unit changes. Both the first collimator and the second
collimator can implement beam collimation. A size of a light spot
area of a beam transmitted to the metasurface is adjusted by using
the beam collimation function of the first collimator, an incident
direction of the beam is controlled so that the beam is transmitted
to a corresponding position on the metasurface, and a phase change
amount of the phase modulation is controlled by designing a
structure of an electromagnetic resonance unit at the corresponding
position, thereby implementing corresponding spatial mode
conversion. A beam obtained after wavefront conversion performed
through the mode division multiplexing module is coupled to an
output fiber in a mode division multiplexing system by using the
beam collimation function of the second collimator, so that
independent signals in an input fiber in the mode division
multiplexing system are multiplexed into corresponding modes in the
output fiber in the mode division multiplexing system, to implement
mode multiplexing.
[0009] Further, a size of the electromagnetic resonance unit is a
sub-wavelength, that is, a feature size of the electromagnetic
resonance unit is smaller than an operating wavelength, and the
size of the electromagnetic resonance unit is smaller than a pixel
size of the spatial light modulator or a phase plate in the prior
art. Therefore, this avoids low resolution in a wavefront
conversion process, ensures comparatively low crosstalk between
different modes, and implements a better signal transmission
effect. In addition, in this embodiment, wavefront conversion
implemented on a beam on the metasurface achieves a smaller
insertion loss and a smaller energy loss of the beam than wavefront
conversion performed on the beam through the phase plate or the
spatial light modulator in the prior art.
[0010] Further, compared with the spatial light modulator in the
prior art, the metasurface in this application may be prepared on a
large scale at low costs by using a technology such as
photolithography or nanoimprint, so that costs of the mode division
multiplexer can be reduced. In addition, wavefront conversion
performed on a beam through the spatial light modulator, in
particular a liquid crystal spatial light modulator in the prior
art is related to a polarization direction of the beam, and
consequently, a problem of polarization dependency occurs. Compared
with the spatial light modulator in the prior art, the mode
division multiplexing module in this application has no problem of
polarization dependency; in other words, the metasurface in this
application can implement polarization-independent phase
modulation.
[0011] In this application, the size of the electromagnetic
resonance unit on the metasurface matches a phase change value
before and after the phase modulation of the beam is performed on
the metasurface, and distribution of the electromagnetic resonance
units of different sizes matches optical field distribution of the
beam on the metasurface.
[0012] A size of an electromagnetic resonance unit on each
metasurface matches a phase change value of the phase modulation
performed on the beam on the metasurface. In other words, the size
of the electromagnetic resonance unit on the metasurface can be
designed based on the phase change value needed for the phase
modulation performed on the beam on the metasurface. For example,
in some embodiments, when the phase change value of the beam needs
to be adjusted to be larger, for example, the beam needs to enabled
to be converted from a fundamental mode state to a higher-order
spatial mode, a size of an electromagnetic resonance unit at a
corresponding position on which the beam is incident on the
metasurface can be appropriately increased. Distribution of the
electromagnetic resonance units of different sizes matches optical
field distribution of the beam on the metasurface, that is, matches
a position at which the beam is incident on the metasurface, so
that the beam has different phase change amounts at different
incident positions. In this application, sizes of the
electromagnetic resonance units at different positions on the
metasurface are designed based on the optical field distribution of
the beam at different positions on the metasurface and phase change
amounts needed for irradiating the beam at different positions on
the metasurface, so that beams transmitted through different
channels can be converted into beams with needed modes, to
implement signal multiplexing and demultiplexing.
[0013] In some embodiments of this application, mode conversion
satisfies the following formula:
O=F.sub.L2T.sup.n.sub.a.times.bF.sub..DELTA.xnT.sup.n-1.sub.a.times.b
. . .
F.sub..DELTA.x2T.sup.2.sub.a.times.bF.sub..DELTA.x1T.sup.1.sub.a.time-
s.bF.sub.L1I
[0014] I is an input optical field distribution matrix (namely, an
optical field distribution matrix output by the first collimator),
and O is an output optical field distribution matrix (namely, an
optical field distribution matrix at a front end of the second
collimator). L1 is a distance of transmitting a beam from the first
collimator to the mode division multiplexing module, L2 is a
distance of transmitting the beam from the mode division
multiplexing module to the second collimator, and F.sub.L1 and
F.sub.L2 respectively indicate Fresnel diffraction matrices
corresponding to the transmission distances L1 and L2.
F.sub..DELTA.xi indicates a Fresnel diffraction matrix
corresponding to a transmission distance .DELTA.x.sub.i, the
distance .DELTA.x.sub.i is a distance of transmitting the beam
after i.sup.th phase modulation and before (i+1).sup.th phase
modulation, i=1, 2, . . . , n, and n is a natural number greater
than 1. T.sup.i.sub.a.times.b is a unitary matrix corresponding to
the i.sup.th phase modulation performed on the metasurface,
a.times.b indicates that each phase modulation is completed by
using a.times.b pixels, each pixel has one or more electromagnetic
resonance units disposed in an array, and the electromagnetic
resonance units in each pixel are of a same structure, that is, the
electromagnetic resonance units in each pixel are made of a same
material and are of a same shape and a same size. An area of the
metasurface including the a.times.b pixels is greater than an
effective light spot area in a transmission process, so that the
beam can be incident on the metasurface.
[0015] A unitary matrix T.sup.i.sub.a.times.b corresponding to each
phase modulation performed on the metasurface is obtained by using
the determined I, O, F.sub.L1, F.sub.L2, and F.sub..DELTA.xi, to
obtain the sizes and the distribution of the electromagnetic
resonance units on the metasurface.
[0016] In this application, the unitary matrix
T.sup.i.sub.a.times.b corresponding to each phase modulation
performed on the metasurface is obtained by using the mode
conversion formula, to obtain a size of phase modulation
corresponding to each pixel, and then determine a size of the
electromagnetic resonance unit in each pixel and the distribution
of the electromagnetic resonance units on the metasurface, to meet
a mode conversion requirement of the mode division multiplexer.
[0017] In some embodiments of this application, the mode division
multiplexer is a linearly polarized mode multiplexer, and a
quantity N of multiplexing modes of the linearly polarized mode
division multiplexer and a quantity (m+1) of times of phase
modulation performed on the metasurface satisfy a formula: m=2N, so
that a good phase modulation effect is implemented through a proper
quantity of times of phase modulation.
[0018] In some other embodiments of this application, the mode
division multiplexer is a non-linearly polarized mode multiplexer,
and a quantity of multiplexing modes of the non-linearly polarized
mode multiplexer is positively related to a quantity of times of
phase modulation performed on the metasurface, that is, a larger
quantity of multiplexing modes indicates a larger quantity of times
of phase modulation performed on the metasurface, so that a good
phase modulation effect is obtained. For example, the mode division
multiplexer may be another non-linearly polarized mode multiplexer
such as a Hermitian-Gaussian mode (HG mode) mode multiplexer.
[0019] In some embodiments of this application, both the first
collimator and the second collimator are metasurfaces. Sizes and
distribution of electromagnetic resonance units on the metasurface
are designed, so that the metasurface has the beam collimation
function, to adjust a size of a light spot of a beam incident on
the metasurface in the mode division multiplexing module through
the first collimator, and a beam output by the mode division
multiplexing module can be coupled to the output fiber after being
collimated through the second collimator. In some other embodiments
of this application, the first collimator and the second collimator
may be collimation lenses, to implement the beam collimation
function.
[0020] Further, in this application, the electromagnetic resonance
unit enables an adjustment range of a phase change value generated
for a beam transmitted to the electromagnetic resonance unit to be
0 to 2.pi., to ensure that the metasurface can effectively adjust a
phase of the beam to any needed value.
[0021] In some embodiments of this application, the mode division
multiplexing module includes a plurality of metasurfaces, the
plurality of metasurfaces are disposed in parallel and spaced from
each other, a beam sequentially passes through the plurality of
metasurfaces, and phase modulation is generated once each time the
beam passes through each metasurface, to implement mode conversion
and multiplexing through a plurality of times of phase modulation,
and reduce a phase change gradient after each phase modulation,
thereby reducing a requirement for each metasurface. In these
embodiments, each metasurface includes a.times.b pixels, so that
each phase modulation is completed by using the a.times.b pixels,
an area of the metasurface including the a.times.b pixels is
greater than an effective light spot area in a transmission
process, and the distance .DELTA.x.sub.i is a distance between an
i.sup.th metasurface and an (i+1).sup.th metasurface.
[0022] In these embodiments, a quantity of times of phase
modulation performed through
[0023] the metasurfaces is the same as a quantity of metasurfaces,
and both are (m+1). When the mode division multiplexer is a
linearly polarized mode division multiplexer, the quantity (m+1) of
metasurfaces and a quantity N of multiplexing modes of the mode
division multiplexer satisfy a formula: m=2N, so that a good phase
modulation effect is implemented through a proper quantity of times
of phase modulation.
[0024] In some embodiments, the metasurface includes a substrate,
an array of the electromagnetic resonance units is disposed on a
surface of the substrate, and a refractive index of a dielectric
material of the electromagnetic resonance unit is greater than 2,
in other words, a refractive index of a dielectric material forming
the electromagnetic resonance unit is greater than 2. Because of
electromagnetic resonance of the electromagnetic resonance unit
made of the high-refractive-index dielectric material, a phase of a
beam passing through the electromagnetic resonance unit can be
easily changed, so that the phase of the beam transmitted through
the electromagnetic resonance unit can be easily changed by
changing the size of the electromagnetic resonance unit.
[0025] In an embodiment of this application, the substrate is a
silicon dioxide substrate, and the electromagnetic resonance unit
is a silicon nanocube. In this embodiment, the electromagnetic
resonance unit is the silicon nanocube, and the silicon nanocube
has a comparatively large refractive index and a comparatively low
energy loss, so that the beam has a comparatively low loss after
passing through the electromagnetic resonance unit, and a needed
phase change can be easily obtained.
[0026] In some other embodiments of this application, the mode
division multiplexing module includes two reflectors disposed
spaced from each other, the beam is emitted after being reflected
by the reflectors for a plurality of times between the two
reflectors, at least one of the two reflectors is the metasurface,
and phase modulation is performed once each time the beam is
reflected on the metasurface. In these embodiments, phase
modulation of the beam can be implemented without disposing a
plurality of metasurfaces spaced from each other, so that a
structure of the mode division multiplexing module can be
simplified, to reduce a volume occupied by the mode division
multiplexer.
[0027] In an embodiment of this application, one of the two
reflectors is the metasurface, and the other reflector is a mirror
reflector, the electromagnetic resonance units on the metasurface
are disposed opposite to a reflection surface of the mirror
reflector, and phase modulation is performed once each time the
beam is reflected on the metasurface, so that mode conversion and
multiplexing of the beam are implemented through a plurality of
times of phase modulation performed on the beam on the metasurface.
In these embodiments, the electromagnetic resonance units on the
metasurface include a plurality of sub-regions in a transmission
direction of the beam, and each reflection of the beam on the
metasurface occurs on one sub-region, so that a quantity of times
of phase modulation is the same as a quantity of sub-regions. In
some embodiments, each sub-region includes a.times.b pixels, so
that each phase modulation is completed by using the a.times.b
pixels, and an area of each sub-region is greater than an effective
light spot area in a transmission process. In these embodiments,
the quantity of times of phase modulation performed on the
metasurface is the same as the quantity of sub-regions, and both
are (m+1). When the mode division multiplexer is a linearly
polarized mode division multiplexer, the quantity (m+1) of
sub-regions and a quantity N of multiplexing modes of the mode
division multiplexer satisfy a formula: m=2N, so that a good phase
modulation effect is implemented through a proper quantity of times
of phase modulation.
[0028] In this embodiment, the metasurface includes a metal
substrate, a dielectric layer, and an array layer that are disposed
sequentially through stacking; the array layer includes a plurality
of metal blocks disposed in an array; the metal substrate includes
a plurality of first sub-blocks disposed in an array; the
dielectric layer includes a plurality of second sub-blocks disposed
in an array; the plurality of first sub-blocks are in a one-to-one
correspondence with the plurality of second sub-blocks; one metal
block is stacked on each second sub-block; and each first
sub-block, each second sub-block, and the metal block stacked on
the second sub-block form the electromagnetic resonance unit. In
addition, in this embodiment, the reflection surface of the mirror
reflector is disposed opposite to the metal blocks, in other words,
the reflection surface of the mirror reflector faces the
metasurface, and the metal blocks on the metasurface face the
mirror reflector.
[0029] In another embodiment, both the two reflectors are
metasurfaces, and phase modulation is performed once each time the
beam is reflected on either of the two metasurfaces, so that mode
conversion and multiplexing of the beam are implemented through a
plurality of times of phase modulation performed on the beam
through the two metasurfacties. In this embodiment, phase
modulation can be performed on both the metasurfaces. Compared with
a mode division multiplexer including a mirror reflector and a
metasurface, a quantity of times of phase modulation that needs to
be performed on a single metasurface is reduced, so that an area of
the metasurface can be further reduced, to further reduce an area
occupied by the mode division multiplexer. Likewise, in this
embodiment, electromagnetic resonance units on the metasurface
include a plurality of sub-regions in a transmission direction of
the beam, and each reflection of the beam occurs on one sub-region,
so that a quantity of times of phase modulation is the same as a
sum of quantities of sub-regions of the two metasurfaces. In some
embodiments, each sub-region includes a.times.b pixels, so that
each phase modulation is completed by using the a.times.b pixels,
and an area of each sub-region is greater than an effective light
spot area in a transmission process. In addition, the quantity of
times of phase modulation performed on the metasurfaces is the same
as the sum of the quantities of sub-regions of the two
metasurfaces, and both are (m+1). When the mode division
multiplexer is a linearly polarized mode division multiplexer, the
quantity (m+1) of sub-regions and a quantity N of multiplexing
modes of the mode division multiplexer satisfy a formula: m=2N, so
that a good phase modulation effect is implemented through a proper
quantity of times of phase modulation.
[0030] In this embodiment, the metasurface includes a metal
substrate, a dielectric layer, and an array layer that are disposed
sequentially through stacking; the array layer includes a plurality
of metal blocks disposed in an array; the metal substrate includes
a plurality of first sub-blocks disposed in an array; the
dielectric layer includes a plurality of second sub-blocks disposed
in an array; the plurality of first sub-blocks are in a one-to-one
correspondence with the plurality of second sub-blocks; one metal
block is stacked on each second sub-block; and each first
sub-block, each second sub-block, and the metal block stacked on
the second sub-block form the electromagnetic resonance unit. In
addition, in this embodiment, metal blocks on the two metasurfaces
are disposed oppositely.
[0031] The electromagnetic resonance unit including the first
sub-block, the second sub-block, and the metal block stacked on the
second sub-block can reflect a beam, and electrical resonance and
magnetic resonance of the electromagnetic resonance unit are
excited when the beam is reflected on the electromagnetic resonance
unit, to effectively implement phase modulation.
[0032] In some embodiments of this application, the metal substrate
is an aluminum substrate, the dielectric layer is a silicon dioxide
layer, and the metal block is a gold block.
[0033] In some embodiments of this application, the mode division
multiplexer further includes an assembly component; and the first
collimator, the second collimator, and the mode division
multiplexing module are all assembled into the assembly component.
The assembly component is configured to protect structures such as
the first collimator, the second collimator, and the mode division
multiplexing module that are located in the assembly component.
Further, in some embodiments, a first optical fiber socket and a
second optical fiber socket are respectively disposed on opposite
sides of the assembly component; the first optical fiber socket is
located on a side close to the first collimator, and the second
optical fiber socket is located on a side close to the second
collimator; and the first optical fiber socket is configured to be
connected to the input fiber, and the second optical fiber socket
is configured to be connected to the output fiber. In this
embodiment, the first optical fiber socket and the second optical
fiber socket are disposed on the assembly component, to facilitate
connection of the input fiber and the output fiber in the mode
division multiplexing system to the mode division multiplexer, and
facilitate application of the mode division multiplexer to the mode
division multiplexing system.
[0034] According to a second aspect, this application provides a
mode division multiplexing system. The mode division multiplexing
system includes an input fiber, an output fiber, and the mode
division multiplexer. The input fiber and the output fiber are
respectively connected to two opposite sides of the mode division
multiplexer. The input fiber is close to a side of a first
collimator in the mode division multiplexer; and the output fiber
is close to a side of a second collimator in the mode division
multiplexer. A beam is sequentially transmitted from the input
fiber to the mode division multiplexer, and then to the output
fiber. The input fiber is configured to provide a plurality of
channels, and each channel transmits one independent beam to the
mode division multiplexer. The output fiber includes a plurality of
different fiber modes, and each fiber mode is used to transmit a
beam of a corresponding spatial mode. The mode division multiplexer
is configured to perform a plurality of times of phase modulation
on a plurality of independent beams input through the input fiber,
so that spatial modes of the plurality of beams are respectively
converted into spatial modes that match different fiber modes in
the output fiber, and combine the plurality of beams. Beams
transmitted through different channels are converted into beams
with different spatial modes through the mode division
multiplexer.
[0035] The input fiber is a standard single-mode fiber array or a
multi-core fiber. The output fiber is a few-mode fiber or a
multi-mode fiber. The input fiber is the standard single-mode fiber
array or the multi-core fiber, so that a plurality of channels are
provided, to output an independent beam through each channel. The
output fiber is the few-mode fiber or the multi-mode fiber, so that
a plurality of different fiber modes are provided, to multiplex
beams in different modes that are obtained after the mode
conversion into the output fiber, thereby implementing space
division multiplexing of the fiber and improving a capacity of
fiber optic communications.
[0036] In this application, independent beams that are transmitted
from different channels and input through the input fiber are
transmitted to the first collimator in the mode division
multiplexer, sizes of light spot areas of the beams transmitted to
the metasurface in the mode division multiplexer are adjusted
through the first collimator, and incident directions are adjusted,
to transmit the beams to corresponding positions on the
metasurface. After mode conversion is performed on the beams by the
mode division multiplexing module, the beams are coupled to the
output fiber through the second collimator, so that independent
signals in the input fiber are multiplexed into corresponding modes
in the output fiber, to implement a mode multiplexing function. In
addition, the mode division multiplexer can implement
high-resolution, low-crosstalk, and low-loss mode conversion, so
that the mode division multiplexing system including the mode
division multiplexer has high-resolution, low-crosstalk, and
low-loss mode conversion, and crosstalk between different modes
multiplexed into the output fiber is low.
[0037] Further, in this application, a few-mode/multi-mode fiber
that supports a specific mode is selected based on a quantity of
single-mode fibers in the standard single-mode fiber array or a
quantity of cores of the multi-core fiber, so that the beams
transmitted through the input fiber can finally be transmitted in
different fiber modes in the output fiber.
[0038] According to a third aspect, this application further
provides a mode division demultiplexing system. The mode division
demultiplexing system includes an input fiber, an output fiber, and
the mode division multiplexer. The input fiber and the output fiber
are respectively connected to two opposite sides of the mode
division multiplexer. The input fiber is close to a side of a
second collimator in the mode division multiplexer; and the output
fiber is close to a side of a first collimator in the mode division
multiplexer. A beam is sequentially transmitted from the input
fiber to the mode division multiplexer, and then to the output
fiber. The input fiber is a few-mode or multi-mode fiber including
a plurality of different fiber modes. The mode division multiplexer
is configured to convert input different spatial modes and perform
beam splitting, so that the input different spatial modes are
converted into spatial modes that match a mode field of the output
fiber. The output fiber is configured to receive and transmit each
split beam emitted through the mode division multiplexer, the
output fiber includes a plurality of channels, and each channel is
configured to transmit one independent split beam.
[0039] The input fiber is the few-mode fiber or the multi-mode
fiber, and has a plurality of different fiber modes; and the output
fiber is a standard single-mode fiber array or a multi-core fiber,
so that a plurality of independent channels disposed in parallel
are provided, to transmit an independent signal through each
channel.
[0040] In this application, a beam input through the input fiber is
transmitted to the second collimator in the mode division
multiplexer, and is transmitted to a mode division multiplexing
module through the second collimator. The beam is split into a
plurality of independent split beams by the mode division
multiplexing module, and simultaneously a spatial mode of each
split beam is converted by the mode division multiplexing module
into a spatial mode that matches the mode field of the output
fiber, so that the split beam can be transmitted in the output
fiber. In some embodiments of this application, each output fiber
is a standard single-mode fiber, and can be configured to transmit
a signal in a fundamental mode state.
[0041] According to a fourth aspect, this application further
provides a communications system. The communications system
includes the mode division multiplexing system and the mode
division demultiplexing system. The output fiber in the mode
division multiplexing system is the input fiber in the
demultiplexing system. Independent signals in the input fiber are
multiplexed into corresponding modes in the output fiber by the
mode division multiplexing system, and the signals are transmitted
through the output fiber. Then, signals that are transmitted by the
output fiber and that have a plurality of different fiber modes are
demultiplexed into standard single-mode fibers or cores of a
multi-core fiber in the output fiber through the demultiplexing
system.
BRIEF DESCRIPTION OF DRAWINGS
[0042] FIG. 1 is a schematic structural diagram of a mode division
multiplexer according to an embodiment of this application;
[0043] FIG. 2 is a schematic structural diagram of a metasurface of
the mode division multiplexer according to the embodiment in FIG.
1;
[0044] FIG. 3 is a schematic structural diagram of a mode division
multiplexer according to another embodiment of this
application;
[0045] FIG. 4 is a schematic structural diagram of a mode division
multiplexer according to another embodiment of this
application;
[0046] FIG. 5 is a schematic diagram of a split structure of a
metasurface of the mode division multiplexer according to the
embodiment in FIG. 4;
[0047] FIG. 6 is a schematic structural diagram of a single
electromagnetic resonance unit of the mode division multiplexer
according to the embodiment in FIG. 4;
[0048] FIG. 7 is a schematic structural diagram of a mode division
multiplexer according to another embodiment of this
application;
[0049] FIG. 8 is a schematic structural diagram of a mode division
multiplexer according to another embodiment of this
application;
[0050] FIG. 9 is a schematic structural diagram of a mode division
multiplexing system according to an embodiment of this
application;
[0051] FIG. 10 is a schematic structural diagram of a mode division
demultiplexing system according to an embodiment of this
application; and
[0052] FIG. 11 is a schematic structural diagram of a
communications system according to an embodiment of this
application.
DESCRIPTION OF EMBODIMENTS
[0053] The following clearly describes the technical solutions in
the embodiments of this application with reference to the
accompanying drawings in the embodiments of this application.
[0054] This application provides a mode division multiplexer. The
mode division multiplexer is usually used in a space division
multiplexing system (including a mode division multiplexing system,
a mode division demultiplexing system, and the like), to improve a
capacity of optical communication. Refer to FIG. 1 and FIG. 2. This
application provides a mode division multiplexer 100. An arrow
direction in FIG. 1 is a transmission direction of a beam in the
mode division multiplexer 100 in this embodiment. In this
application, an operating band of the mode division multiplexer is
in a fiber optic communications band. The mode division multiplexer
100 includes a first collimator 20, a second collimator 30, and a
mode division multiplexing module 10 located between the first
collimator 20 and the second collimator 30. After mode conversion
and beam combining are performed by the mode division multiplexing
module 10 on a plurality of independent beams incident through the
first collimator 20, a combined beam is incident on the second
collimator 30.
[0055] The mode division multiplexing module 10 includes one or
more metasurfaces 10a disposed in parallel and spaced from each
other. The metasurface 10a is a laminated structure whose thickness
is less than a wavelength, and the metasurface 10a includes
sub-wavelength electromagnetic resonance units 11 disposed in an
array. The electromagnetic resonance unit 11 can change a phase, an
amplitude, a polarization state, and the like of a beam passing
through the electromagnetic resonance unit 11, and a spatial mode
of the beam may be converted through a plurality of times phase
modulation performed on the beam by the mode division multiplexing
module 10. For example, the beam is converted from a fundamental
mode to a higher-order mode, or the beam is converted from a
higher-order mode to a fundamental mode. The electromagnetic
resonance unit 11 is of a sub-wavelength structure, that is, a
feature size of the electromagnetic resonance unit 11 is smaller
than an operating wavelength, and a period of the electromagnetic
resonance unit is approximately half a wavelength of a light wave.
In this application, the period of the electromagnetic resonance
unit is a distance between centers of two adjacent electromagnetic
resonance units. The feature size is a size of representative
significance in sizes of the electromagnetic resonance unit 11. For
example, when the electromagnetic resonance unit 11 is of a
cylindrical structure, the feature size is essentially a height of
the cylindrical structure and a cross-sectional radius of the
cylindrical structure; or when the electromagnetic resonance unit
11 is of a quadrangular prism structure, the feature size is
essentially a height of the quadrangular prism structure and a
cross-sectional length and width of the quadrangular prism
structure. In this embodiment, the electromagnetic resonance units
11 are all quadrangular prism structures. The electromagnetic
resonance unit 11 is of the sub-wavelength structure, that is, the
feature size of the electromagnetic resonance unit 11 is smaller
than the operating wavelength, and is smaller than a pixel size of
a spatial light modulator or a phase plate for phase modulation in
the prior art. Therefore, this avoids low resolution in a wavefront
conversion process, ensures comparatively low crosstalk between
different modes, and implements a better signal transmission
effect. A liquid crystal molecule in the spatial light modulator in
the prior art has a specific polarization direction, and
consequently, a problem of polarization dependency occurs on the
spatial light modulator in the prior art. However, the metasurface
10a in this application can implement polarization-independent
phase modulation. In addition, in this embodiment, wavefront
conversion implemented on a beam on the metasurface 10a achieves a
smaller insertion loss and a smaller energy loss of the beam than
wavefront conversion performed on the beam through the spatial
light modulator in the prior art.
[0056] Further, compared with the spatial light modulator in the
prior art, the metasurface 10a in this application may be prepared
on a large scale at low costs by using a technology such as
photolithography or nanoimprint, so that preparation costs of the
mode division multiplexer 100 can be reduced.
[0057] In this application, a size of an electromagnetic resonance
unit 11 in each metasurface 10a matches a phase change value of
phase modulation performed on the beam on the metasurface 10a. In
other words, the size of the electromagnetic resonance unit 11 can
be designed based on the phase change value needed by the beam, to
implement corresponding phase modulation. For example, in some
embodiments, when the phase change value of the beam needs to be
adjusted to be larger, for example, the beam needs to enabled to be
converted from a fundamental mode state to a higher-order spatial
mode, the size of the electromagnetic resonance unit 11 may be
appropriately increased. In addition, distribution of the
electromagnetic resonance units 11 of different sizes matches
optical field distribution of the beam on the metasurface 10a, that
is, matches a light spot position at which the beam is incident on
the metasurface 10a, so that the beam has different phase change
amounts at different light spot positions at which the beam is
incident on the metasurface 10a, to implement different phase
modulation. In this application, needed phase change amounts are
determined based on the optical field distribution of the beam at
different positions on the metasurface 10a, and then sizes of the
electromagnetic resonance units 11 at different positions on the
metasurface 10a are designed, so that beams transmitted through
different channels can be converted into beams with specific modes,
thereby implementing signal multiplexing and demultiplexing.
[0058] In some embodiments of this application, mode conversion
satisfies the following formula:
O=F.sub.L2T.sup.n.sub.a.times.bF.sub..DELTA.xnT.sup.n-1.sub.a.times.b
. . .
F.sub..DELTA.x2T.sup.2.sub.a.times.bF.sub..DELTA.x1T.sup.1.sub.a.time-
s.bF.sub.L1I
[0059] I is an input optical field distribution matrix (namely, an
optical field distribution matrix that is of a beam and that is
output through the first collimator 20), and O is an output optical
field distribution matrix (namely, an optical field distribution
matrix existing before the beam is input to the second collimator
30). L1 is a distance of transmitting the beam from the first
collimator 20 to the mode division multiplexing module 10, L2 is a
distance of transmitting the beam from the mode division
multiplexing module 10 to the second collimator 30, and F.sub.L1
and F.sub.L2 respectively indicate Fresnel diffraction matrices
corresponding to the distances L1 and L2. F.sub..DELTA.xi indicates
a Fresnel diffraction matrix corresponding to a transmission
distance .DELTA.x.sub.i, the distance .DELTA.x.sub.i is a distance
of transmitting the beam after i.sup.th phase modulation and before
(i+1).sup.th phase modulation, i=1, 2, . . . , n, and n is a
natural number greater than 1. T.sup.i.sub.a.times.b is a unitary
matrix corresponding to the i.sup.th phase modulation performed on
the metasurface, a.times.b indicates that each phase modulation is
completed by using a.times.b pixels, each pixel has one or more
electromagnetic resonance units 11 disposed in an array, and the
electromagnetic resonance units 11 in each pixel are of a same
structure, that is, the electromagnetic resonance units 11 in each
pixel are made of a same material and are of a same shape and a
same size. An area of the metasurface including the a.times.b
pixels is greater than an effective light spot area in a
transmission process.
[0060] T.sup.i.sub.a.times.b matches electromagnetic resonance
units 11 on a metasurface 10a corresponding to the i.sup.th phase
modulation, that is, the electromagnetic resonance units 11 provide
phase modulation of corresponding pixels. F.sub..DELTA.xi indicates
the Fresnel diffraction matrix corresponding to the passing
distance .DELTA.x.sub.i, and the distance .DELTA.x.sub.i is a
distance between an i.sup.th metasurface 10a and an (i+1).sup.th
metasurface 10a.
[0061] A unitary matrix T.sup.i.sub.a.times.b corresponding to each
phase modulation performed on the metasurface 10a is obtained by
using the determined I, O, F.sub.L1, F.sub.L2, and F.sub..DELTA.xi,
and a value of each numerical point in the unitary matrix
T.sup.t.sub.a>.sub.t is a phase change amount of a pixel at a
corresponding position on the metasurface. For example, if a value
in an i.sup.th row and a i.sup.th column of the unitary matrix
T.sup.i.sub.a.times.b is p, a phase change amount that needs to be
generated when the beam is transmitted to a pixel in an i.sup.th
row and a j.sup.th column on the metasurface 10a is p. Therefore,
after the unitary matrix T.sup.i.sub.a.times.b is obtained
according to the mode conversion formula, a phase change amount
that needs to be generated through phase modulation performed on
the beam at each of the a.times.b pixels is learned, so that
distribution of the pixels of the metasurface 10a and sizes and
distribution of electromagnetic resonance units 11 in each pixel
can be designed, to obtain, through design, a metasurface 10a that
meets an actual requirement. In other words, in this application,
the metasurface 10a that meets the actual requirement can be easily
obtained through design by using the mode conversion formula, which
is simple and convenient.
[0062] Further, in this application, the electromagnetic resonance
unit 11 enables a range of a phase change value generated for a
beam transmitted to the electromagnetic resonance unit 11 to be 0
to 2.pi., to ensure that the metasurface 10a can effectively adjust
a phase of the beam to any needed value.
[0063] In this application, a quantity of times of phase modulation
performed on the beam on the metasurface 10a is set in a specific
linear relationship with a quantity of multiplexing modes of the
mode division multiplexer 100, so that a good phase modulation
effect can be implemented through a proper quantity of times of
phase modulation. In some embodiments of this application, the mode
division multiplexer 100 is a linearly polarized mode division
multiplexer, and a quantity N of multiplexing modes of the linearly
polarized mode division multiplexer 100 and the quantity (m+1) of
times of phase modulation performed on the metasurface 10a satisfy
a formula: m=2N. In some other embodiments of this application, the
mode division multiplexer 100 is anon-linearly polarized mode
multiplexer, and a quantity of multiplexing modes of the
non-linearly polarized mode multiplexer is positively related to
the quantity of times of phase modulation performed on the
metasurface 10a, that is, a larger quantity of multiplexing modes
indicates a larger quantity of times of phase modulation performed
on the metasurface 10a, so that a good phase modulation effect is
obtained. In this application, the mode division multiplexer 100
may be another non-linearly polarized mode multiplexer such as a
Hermitian-Gaussian mode (HG mode) mode multiplexer.
[0064] In this application, the mode division multiplexing module
10 may be of a transmissive or reflective structure.
[0065] In this embodiment, the mode division multiplexing module 10
is transmissive, and a beam passes through the metasurface 10a to
implement corresponding phase modulation. The mode division
multiplexing module 10 includes a plurality of metasurfaces 10a,
and the plurality of metasurfaces 10a are disposed oppositely in
parallel. Phase modulation is generated once when the beam passes
through each metasurface 10a, to convert a mode of the beam through
a plurality of times of phase modulation, so that a gradient of
phase modulation performed on the beam through a single metasurface
10a is comparatively small, thereby reducing a requirement for each
metasurface 10a. In this embodiment, each metasurface 10a may be
split into a.times.b pixels, so that each phase modulation is
completed by using the a.times.b pixels, and an area of the
metasurface including the a.times.b pixels is greater than an
effective light spot area in a transmission process. In this
embodiment, both a central axis of the first collimator 20 and a
central axis of the second collimator 30 are perpendicular to a
plane in which the metasurface 10a is located, and the transmission
direction of the beam is perpendicular to the plane in which the
metasurface 10a is located. In this embodiment, the distance
between the i.sup.th metasurface and the (i+1).sup.th metasurface
is the distance .DELTA.x.sub.i in the mode conversion formula. The
metasurfaces 10a can be arranged at equal intervals or at non-equal
intervals. In this embodiment, the metasurfaces 10a are arranged at
equal intervals, in other words, .DELTA.x.sub.1=.DELTA.x.sub.2 . .
. =.DELTA.x.sub.n.
[0066] In this embodiment, because phase modulation is generated
once when the beam passes through each metasurface 10a, a quantity
of times of phase modulation performed on the metasurfaces is the
same as a quantity of metasurfaces 10a. Therefore, when the mode
division multiplexer is a linearly polarized mode division
multiplexer, the quantity (m+1) of metasurfaces 10a and a quantity
N of multiplexing modes of the mode division multiplexer 100
satisfy a formula: m=2N, so that a good phase modulation effect is
implemented through a proper quantity of times of phase
modulation.
[0067] In this embodiment, the metasurface 10a further includes a
substrate 12, and the electromagnetic resonance units 11 are
disposed on a surface of the substrate 12. In addition, in this
embodiment, the substrate 12 includes a first surface and a second
surface that are opposite to each other, and first surfaces of the
metasurfaces 10a face a same side. The electromagnetic resonance
units 11 can be located on the first surface or the second surface.
In this embodiment, each metasurface 10a is located on the first
surface.
[0068] In this embodiment, a material forming the electromagnetic
resonance unit 11 is a dielectric material having a comparatively
large refractive index, and a beam has a comparatively low energy
loss after passing through the electromagnetic resonance unit 11.
In addition, in this embodiment, the refractive index of the
dielectric material forming the electromagnetic resonance unit 11
is greater than 2. Because of electromagnetic resonance of the
electromagnetic resonance unit 11 made of the high-refractive-index
dielectric material, a phase of a beam passing through the
electromagnetic resonance unit 11 can be easily changed, so that
the phase of the beam transmitted through the electromagnetic
resonance unit 11 can be easily changed by changing the size of the
electromagnetic resonance unit 11.
[0069] In this embodiment, the substrate 12 may be a silicon
dioxide substrate, and the electromagnetic resonance unit 11 may be
a silicon nanocube. The silicon nanocube is in a shape of a
quadrangular prism, and a cross-section that is of the silicon
nanocube and that is parallel to the substrate 12 is a square with
an edge length W. It may be understood that, in another embodiment
of this application, the electromagnetic resonance unit 11 may
alternatively be of another block structure, such as a cylindrical
structure or a multi-prism structure. In this embodiment, the
electromagnetic resonance units 11 have a same height H, and a
phase change value of a beam passing through the metasurface 10a is
controlled by controlling the edge length W of the cross-section of
the silicon nanocube. It may be understood that, in another
embodiment of this application, the substrate 12 and the
electromagnetic resonance unit 11 may be made of other materials.
For example, the electromagnetic resonance unit 11 may be in a
shape of a block made of a material such as silicon nitride.
[0070] In some embodiments of this application, the mode division
multiplexer 100 further includes an assembly component, and the
first collimator 20, the second collimator 30, and the mode
division multiplexing module 10 are all assembled into the assembly
component to protect structures such as the first collimator 20,
the second collimator 30, and the mode division multiplexing module
10 that are located in the assembly component.
[0071] Further, in some embodiments, a first optical fiber socket
and a second optical fiber socket are respectively disposed on two
opposite sides of the assembly component, the first optical fiber
socket is located on a side close to the first collimator 20, the
second optical fiber socket is located on a side close to the
second collimator 30, the first optical fiber socket is configured
to be connected to an input fiber, and the second optical fiber
socket is configured to be connected to an output fiber. In this
embodiment, the first optical fiber socket and the second optical
fiber socket are disposed on the assembly component, to facilitate
connection of the input fiber and the output fiber in a mode
division multiplexing system to the mode division multiplexer 100,
and facilitate application of the mode division multiplexer 100 to
the mode division multiplexing system.
[0072] The first collimator 20 is configured to control a size of a
light spot area of a beam transmitted to the metasurface 10a. The
second collimator 30 is configured to couple a beam emitted through
the mode division multiplexing module 10 to the output fiber for
transmission. In this embodiment, the first collimator 20 and the
second collimator 30 each include a microlens 21; and the first
collimator 20 and the second collimator 30 each may include a
single microlens 21, or may include a lens array including a
plurality of microlenses 21. In this embodiment, the first
collimator 20 includes a lens array including a plurality of
microlenses 21, each microlens 21 corresponds to one incident beam,
and each beam can be transmitted to the mode division multiplexing
module 10 through a corresponding microlens, so that the beam is
more precisely adjusted, to obtain an input optical field
distribution matrix that meets a requirement. The second collimator
30 is a microlens 21.
[0073] In another embodiment of this application, for example, in
an embodiment shown in FIG. 3, both the first collimator 20 and the
second collimator 30 may be metasurfaces. A metasurface that is of
the first collimator 20 has a beam collimation function by
designing sizes and distribution of electromagnetic resonance units
on the metasurface. A size of a light spot of a beam incident on
the metasurface through the first collimator 20 is adjusted by
using the beam collimation function of the first collimator 20. The
second collimator 30 is enabled to couple, to the output fiber by
using a collimation function of the second collimator 30, a beam
output through the mode division multiplexing module 10.
[0074] FIG. 4 provides another mode division multiplexer 200
according to this application. A mode division multiplexing module
10 in the mode division multiplexer 200 is of a reflective
structure, that is, a beam is reflected when being transmitted to a
metasurface 10a in the mode division multiplexing module 10.
Specifically, a difference between the mode division multiplexer
200 and the mode division multiplexer 100 lies in that the mode
division multiplexing module 10 includes two reflectors that are
disposed spaced from each other, a beam is emitted after being
reflected by the reflectors for a plurality of times between the
two reflectors, at least one of the two reflectors is the
metasurface 10a, and phase modulation is performed once each time
the beam is reflected on the metasurface 10a. In these embodiments,
compared with the mode division multiplexing module 100, phase
modulation of the beam can be implemented without disposing a
plurality of metasurfaces 10a spaced from each other, so that a
structure of the mode division multiplexing module 10 in the mode
division multiplexer 200 can be simplified, to reduce a volume
occupied by the mode division multiplexer 200.
[0075] Specifically, in this embodiment, one of the two reflectors
is the metasurface 10a, and the other reflector is a mirror
reflector 40. The mirror reflector 40 includes a reflection surface
41 that can reflect a beam, and the reflection surface 41 of the
mirror reflector 40 faces the metasurface 10a, so that the beam is
transmitted to the metasurface 10a after being reflected by the
mirror reflector 40. Phase modulation is performed once each time
the beam is reflected on the metasurface 10a. In addition, a
central axis of the first collimator 20 and a central axis of the
second collimator 30 each are at an angle .theta. with the
metasurface 10a, so that the beam can be incident on the
metasurface 11 and transmitted to a side of the second collimator
30. The beam is emitted after being reflected for a plurality of
times between the mirror reflector 40 and the metasurface 10a.
Phase modulation is generated once each time the beam is reflected
on the metasurface 10a, and mode conversion and multiplexing of the
beam are implemented after a plurality of times of phase
modulation. In this embodiment, there is no need to dispose a
plurality of metasurfaces 10a spaced from each other, so that the
structure of the mode division multiplexing module 10 in the mode
division multiplexer 200 is simplified, to reduce the volume
occupied by the mode division multiplexer 200. The metasurface 10a
has a plurality of sub-regions disposed adjacently, the sub-regions
are sequentially disposed in a transmission direction of the beam,
and each reflection of the beam occurs on one sub-region. Each
sub-region includes a.times.b pixels, so that each phase modulation
is completed by using the a.times.b pixels, and an area of each
sub-region is greater than an effective light spot area in a
transmission process. In this embodiment, the metasurface 10a is
disposed in parallel with the reflection surface 41, and a distance
between a surface that is of the metasurface 10a and on which
electromagnetic resonance units 11 are disposed and the reflection
surface 41 is Di, so that there is a same distance between
positions at which any two adjacent times of reflection of the beam
occur on the metasurface 10a, the distance between the positions at
which two adjacent times of reflection occur is D.sub.2, and
D.sub.2=2D.sub.1/tan .theta.. In this embodiment, the distance
.DELTA.x.sub.i=2D.sub.1/sin .theta. in the mode conversion
formula.
[0076] In this embodiment, because each reflection of the beam
occurs on one sub-region, and phase modulation is generated once
each time the beam is reflected on the metasurface 10a, a quantity
of times of phase modulation performed on the metasurface is the
same as a quantity of sub-regions of the metasurface 10a.
Therefore, when the mode division multiplexer is a linearly
polarized mode division multiplexer, the quantity (m+1) of
sub-regions and a quantity N of multiplexing modes of the mode
division multiplexer satisfy the following formula: m=2N, so that a
good phase modulation effect is implemented through a proper
quantity of times of phase modulation.
[0077] Refer to FIG. 5 and FIG. 6. In this embodiment, the
metasurface 10a in the mode division multiplexer 200 includes a
metal substrate 113, a dielectric layer 114, and an array layer
that are disposed sequentially through stacking. The array layer
includes a plurality of metal blocks 115 disposed in an array. The
metal substrate 113 includes a plurality of first sub-blocks 113a
disposed in an array. The dielectric layer 114 includes a plurality
of second sub-blocks 114a disposed in an array. The plurality of
first sub-blocks 113a are in a one-to-one correspondence with the
plurality of second sub-blocks 114a, that is, an orthographic
projection of each second sub-block 114a on the metal substrate 113
coincides with one first sub-block 113a. In addition, one metal
block 115 is stacked on each second sub-block 114a; and each first
sub-block 113a, each second sub-block 114a, and each metal block
115 disposed on the second sub-block 114a form the electromagnetic
resonance unit 11. In this embodiment, a period of the
electromagnetic resonance unit 11 is the same as an edge length of
the first sub-block 113a or the second sub-block 114a. The
electromagnetic resonance unit 11 including the first sub-block
113a, the second sub-block 114a, and the metal block 115 disposed
on the second sub-block 114a can reflect a beam, and corresponding
electrical resonance and magnetic resonance are excited when the
beam is incident on the electromagnetic resonance unit 11, to
generate a phase change. In this embodiment, the reflection surface
41 is disposed opposite to the metal blocks 115, in other words,
the reflection surface 41 faces the metasurface 10a, and the metal
blocks 115 on the metasurface 10a face the mirror reflector 40. In
an embodiment of this application, the metal substrate 113 is an
aluminum substrate, the dielectric layer 114 is a silicon dioxide
layer, and the metal block 115 is a gold block. It may be
understood that in this application, the metal substrate 113, the
dielectric layer 114, and the metal block 115 may be made of other
materials. For example, the metal block 115 may alternatively be
made of a material such as aluminum or silver.
[0078] It may be understood that structures of the first collimator
20 and the second collimator 30 in the mode division multiplexer
200 are the same as structures of the first collimator 20 and the
second collimator 30 in the mode division multiplexer 100, and each
may be of a structure including the microlens 21, or may be of a
structure including the metasurface. In the embodiment shown in
FIG. 7, the first collimator 20 and the second collimator 30 in the
mode division multiplexer 200 each are of a structure including the
metasurface.
[0079] Refer to FIG. 8. Another embodiment of this application
provides a mode division multiplexer 300. A difference between the
mode division multiplexer 300 and the mode division multiplexer 200
lies in that two reflectors in the mode division multiplexer 300
each are the metasurface 10a, and phase modulation is performed
once each time a beam is reflected on either of the two
metasurfaces 10a, so that mode conversion and multiplexing of the
beam are implemented through a plurality of times of phase
modulation performed on the beam through the two metasurfaces 10a.
In addition, in this embodiment, phase modulation can be performed
on both the two metasurfaces 10a. Compared with the mode division
multiplexer 200, a quantity of times of phase modulation that needs
to be performed on a single metasurface 10a is reduced, so that an
area of the metasurface 10a in the mode division multiplexer 300 in
this embodiment can be further reduced, to reduce an occupation
area of the mode division multiplexer 300. Likewise, in this
embodiment, electromagnetic resonance units 11 on the metasurface
10a include a plurality of sub-regions in a transmission direction
of the beam, and each reflection of the beam occurs on one
sub-region, so that a quantity of times of phase modulation is the
same as a sum of quantities of sub-regions of the two metasurfaces
10a. In some embodiments, each sub-region includes a.times.b
pixels, so that each phase modulation is completed by using the
a.times.b pixels, and an area of each sub-region is greater than an
effective light spot area in a transmission process. In addition,
the quantity of times of phase modulation performed on the
metasurfaces is the same as the sum of the quantities of
sub-regions of the two metasurfaces 10a, and both are (m+1).
Therefore, when the mode division multiplexer 300 is a linearly
polarized mode division multiplexer, the quantity (m+1) of
sub-regions and a quantity N of multiplexing modes of the mode
division multiplexer 300 satisfy the following formula: m=2N, so
that a good phase modulation effect is implemented through a proper
quantity of times of phase modulation.
[0080] In this embodiment, the two metasurfaces 10a are disposed in
parallel, a distance between the two metasurfaces 10a is D.sub.1, a
distance between positions at which two adjacent times of
reflection occur on a same metasurface is D.sub.2, and
D.sub.2=2D.sub.1/tan .theta.. In this embodiment, the distance
.DELTA.x.sub.i=D.sub.1/sin .theta. in the mode conversion
formula.
[0081] Further, in this embodiment, a structure of the metasurface
10a in the mode division multiplexer 300 is the same as a structure
of the metasurface 10a in the mode division multiplexer 200, and
metal blocks 115 on the two metasurfaces 10a in the mode division
multiplexer 300 are disposed oppositely, to ensure that the
metasurface 10a has a phase modulation function while implementing
a reflection effect.
[0082] Refer to FIG. 9. This application provides a mode division
multiplexing system 1000. The mode division multiplexing system
1000 is configured to convert and multiplex a transmission mode in
a single-mode fiber array or a multi-core fiber, so that signals in
channels are multiplexed into a same few-mode fiber or a multi-mode
fiber. The mode division multiplexing system 1000 includes an input
fiber 110, an output fiber 120, and the mode division multiplexer
100. It may be understood that in another embodiment of this
application, the mode division multiplexer may alternatively be the
mode division multiplexer 200, the mode division multiplexer 300,
or a mode division multiplexer in another embodiment of this
application. The input fiber 110, the mode division multiplexer
100, and the output fiber 120 are sequentially located on a
propagation path of a beam. The input fiber and the output fiber
are respectively connected to two opposite sides of the mode
division multiplexer 100. The input fiber is close to a side of the
first collimator 20 in the mode division multiplexer 100. The
output fiber is close to a side of the second collimator 30 in the
mode division multiplexer 100. The beam is sequentially transmitted
from the input fiber to the mode division multiplexer 100, and then
to the output fiber.
[0083] The input fiber 110 is configured to provide a plurality of
channels. In this embodiment, the input fiber 110 is a standard
single-mode fiber array or a multi-core fiber, and each single-mode
fiber or each core provides one channel for signal transmission.
Each channel transmits one independent beam to the mode division
multiplexing module 10 through the first collimator 20. The first
collimator 20 controls a size of a light spot area of the beam
transmitted to the metasurface 10a. In this application, a size of
a light spot of the beam transmitted to the metasurface 10a is
adjusted through the first collimator 20, and an arrangement manner
of fibers or cores in the input fiber 110 affects the optical field
distribution matrix I output by the collimator 20.
[0084] The output fiber 120 is a few-mode fiber or a multi-mode
fiber, and can provide a plurality of different fiber modes. Beams
in different modes that are obtained after the mode conversion are
multiplexed into the output fiber, to implement space division
multiplexing of a signal and improve a capacity of fiber optic
communications. In addition, positions of the output fiber 120 and
the mode division multiplexer are fixed, so that positions of the
output fiber 120 and the metasurface 10a are fixed. Therefore, the
output optical field distribution matrix O can be determined.
[0085] The mode division multiplexer 100 is configured to perform
phase modulation for a plurality of times on a plurality of
independent beams input by the input fiber 110, so that spatial
modes of the plurality of beams are respectively converted into
spatial modes that match different fiber modes in the output fiber
120, and combine the plurality of beams. Beams transmitted through
different channels are converted into beams with different spatial
modes through the mode division multiplexer 100. For example, in an
embodiment, the beams transmitted through the input fiber 110 are
all in a fundamental linearly polarized (LP) mode (namely,
LP.sub.01 mode), and the output fiber 120 supports fiber modes such
as LP.sub.11, LP.sub.02, and LP.sub.12, where the subscripts are
angular orders and radial orders of the modes. Therefore, the
plurality of beams in the LP.sub.01 mode are converted into beams
with modes such as LP.sub.11, LP.sub.02, and LP.sub.12 through the
mode division multiplexer 100, and after the plurality of beams are
combined through the mode division multiplexer 100, a combined beam
is coupled to the output fiber 120.
[0086] In this application, independent beams that are transmitted
from different channels and input through the input fiber 110 are
transmitted to the first collimator 20, and a size of a light spot
area of the beam transmitted to the metasurface 10a in the mode
division multiplexer 100 is adjusted through the first collimator
20. After mode conversion and beam combining are performed by the
mode division multiplexer 100 on the plurality of beams input
through the first collimator 20, the combined beam is coupled to
the output fiber 120 through the second collimator 30, so that
independent signals in the input fiber 110 are multiplexed into
corresponding modes in the output fiber 120, thereby implementing a
mode multiplexing function. In addition, the mode division
multiplexer 100 can implement high-resolution, low-crosstalk, and
low-loss wavefront conversion, so that the mode division
multiplexing system including the mode division multiplexer 100 has
high-resolution, low-crosstalk, and low-loss features, and
crosstalk between fiber modes multiplexed into the output fiber 120
is low.
[0087] In this application, a few-mode/multi-mode fiber that
supports a specific mode is selected based on a quantity of
single-mode fibers in the standard single-mode fiber array or a
quantity of cores of the multi-core fiber, so that a plurality of
signals transmitted through the input fiber 110 can finally be
transmitted in different fiber modes in the output fiber 120. Refer
to FIG. 10. This application further provides a mode division
demultiplexing system 2000. The mode division demultiplexing system
2000 is configured to convert different transmission modes in a
same fiber and split a beam, and separately transmit split beams
through independent fibers. In this application, a structure of the
mode division demultiplexing system 2000 is the same as a structure
of the mode division multiplexing system 1000; and a difference
between the structure of the mode division demultiplexing system
2000 and the structure of the mode division multiplexing system
1000 lies in that a transmission direction of a beam in the mode
division demultiplexing system 2000 is opposite to a transmission
direction of a beam in the mode division multiplexing system
1000.
[0088] Specifically, the mode division demultiplexing system 2000
includes an input fiber 210, an output fiber 220, and the mode
division multiplexer 100. The input fiber 210 and the output fiber
220 are respectively connected to two opposite sides of the mode
division multiplexer 100. The input fiber 210 is close to a side of
the second collimator 30 in the mode division multiplexer 100. The
output fiber 220 is close to a side of the first collimator 20 in
the mode division multiplexer 100. A beam is sequentially
transmitted from the input fiber 210 to the mode division
multiplexer 100, and then to the output fiber 220. In another
embodiment of this application, the mode division demultiplexing
may alternatively be the mode division multiplexer 200, the mode
division multiplexer 300, or a mode division multiplexer in another
embodiment of this application. The input fiber 210 is configured
to transmit signals in different fiber modes, to improve a capacity
of fiber optic communications. The input fiber 210 supports a
plurality of different fiber modes, and each fiber mode corresponds
to one type of specific optical field distribution. The mode
division multiplexer 100 splits a beam input through the input
fiber 210 into a plurality of independent split beams and performs
corresponding mode conversion, so that optical field distribution
of a spatial mode of each split beam matches a mode field of the
output fiber 220 after the conversion.
[0089] The input fiber 210 is a few-mode fiber or a multi-mode
fiber, and has a plurality of different fiber modes. The output
fiber 220 is a standard single-mode fiber array or a multi-core
fiber, so that a plurality of independent channels disposed in
parallel are provided, to transmit an independent signal through
each channel.
[0090] In this embodiment, the beam input through the input fiber
210 is transmitted to the second collimator 30 in the mode division
multiplexer 100, and is transmitted to the mode division
multiplexing module 10 through the second collimator 30. The beam
is split into the plurality of independent split beams by the mode
division multiplexing module 10, and simultaneously the spatial
mode of each split beam is converted into the spatial mode that
matches the mode field of the output fiber 220, so that the split
beam can be transmitted in the output fiber. In some embodiments of
this application, each mode supported by the output fiber 220 is a
fundamental linearly polarized mode (namely, LP.sub.01 mode) for
transmitting a signal in a fundamental mode state.
[0091] Refer to FIG. 11. This application further provides a
communications system. The communications system includes the mode
division multiplexing system 1000 and the demultiplexing system
2000. The output fiber 120 in the mode division multiplexing system
1000 is the input fiber 210 in the mode division demultiplexing
system 2000, in other words, the output fiber 120 and the input
fiber 210 are a same fiber. Independent signals in the input fiber
110 are multiplexed into corresponding modes in the output fiber
120 through the mode division multiplexing system 1000, and the
signals are transmitted through the output fiber 120. Then, the
signals that are transmitted by the output fiber 120 and that have
a plurality of different fiber modes are demultiplexed into
standard single-mode fibers or cores in the output fiber 220
through the demultiplexing system 2000.
[0092] The foregoing descriptions are example implementations of
this application. It should be noted that a person of ordinary
skill in the art may make several improvements or polishing without
departing from the principle of this application and the
improvements or polishing shall fall within the protection scope of
this application.
* * * * *